I suspect...but don't know for sure...that there is a mold they do the initial pour of the blue into. That mold would be round (or round-ish), like the blank (probably larger in diameter), but would have the star shapes protruding inward from its edges. So the blue would harden with the spaces for the white stars. Then the white stars would be filled in.
I picked up a couple of patriotic blanks...oh, I forget who actually manufactures them. I'm pretty sure they were made that way, but they were not then subsequently rounded. They are...octagonal or something in shape, and you can fairly clearly see the layering of the resins. I figure the round blanks like this, are probably rounded after the fact to take off the outer layers of white for the star resin, to show the actual nature of the stars in the blank. In the case for my blanks, I really cannot say what the stars look like (or, for that matter, the stripes in the red blank) because its still just the rough "cut".
It could also be that the blanks are poured round at the desired size, with the star shapes (or line or wavy line shapes, or whatever shape) extruded inward from the inside edge of the mold. Then a hold could be bored through the center of the blank, and then filled from the inside out with the secondary color to fill in the initial voids. In either case, the shapes would be a part of the initial mold, and then would be filled in after the base resin was removed from that first mold.